The use of carbamate compounds as prodrugs is well known. Carbamate compounds are well suited for prodrug design because they can be used to regenerate the parent drug whether the point of connection to a vector molecule is a hydroxyl group or an amine. Carbamate prodrugs based on intramolecular cyclizations have been reported since the late 1980s. For example, U.S. Pat. No. 4,812,590 (which corresponds to EP 0 296 811) discloses derivatives of 4-hydroxyanisole carbamate as prodrugs for the delivery and concentration of 4-hydroxyanisole in melanomas. Vigroux et al. (J. Med. Chem. 38, 3983-3994, 1995) disclose prodrugs of acetaminophen which incorporate N-(substituted 2-hydroxyphenyl)- and N-(substituted 2-hydroxypropyl)carbamates.
An optimal prodrug design is one in which the drug is conjugated to a biologically active vector such that the conjugate is completely stable in circulation, but releases the therapeutic agent cleanly when internalized into target cells. Various types of linker technologies attempt to achieve this goal by different chemical means. Two types of biodegradation phenomena, passive hydrolysis and enzymatic hydrolysis, are typically considered. Passive hydrolysis occurs when the molecule degrades by simple chemical decomposition; esters, carbonates, amides and urethanes are all susceptible to varying degrees of hydrolysis with the following order of stability at basic pH: urethanes >amides>carbonates>esters. Enzymatic hydrolysis occurs in plasma circulation as well as upon internalization. If the desired result is to transport the conjugate to the target cell, it is desirable that the conjugate be very resistant to plasma enzymes that could degrade the complex en route. Commonly used ester derivatives are a particularly poor choice of linker because peripherally circulating compound is rapidly attacked by ubiquitous esterases.
Another linker strategy capitalizes on pH differences existing between the inside and outside of cells. Lysosomes, which are responsible for degrading molecules within cells, have a pH of 4-6, while plasma and extracellular fluid has a pH of 7.4. A linker that is designed to degrade at pH 5, but is stable at pH 7, may be useful for delivering the conjugate to cells, assuming that the conjugate does not undergo passive or enzymatic hydrolysis in plasma. However, this linker strategy is not optimal for peptide conjugates because most of the work-up and purification of synthetic peptides is done in acidic media.
Still other chemical linkers attempt to capitalize on specific enzymes found in lysosomes. For example, many peptide sequences are stable in plasma but are specifically cleaved by lysosomal enzymes.
The toxic side effects of many of these therapies, as well as standard treatments of neoplastic disease, effectively limit the amount of active agent that may be administered to a patient. Additionally, many active agents cause organ-specific toxicities, further limiting the dose that may be delivered to the target tissue. For instance, the cardiotoxicity of many anthracycline family members reduces the maximum therapeutic dose available for this group of chemotherapeutic agents. Targeted drug delivery of various therapeutic agents can lower toxicity in normal tissue and increase the efficacy of treatment by allowing concentrated localized effects on specific tissues.
Somatostatin, bombesin, and other biologically active peptide analogs have been used to detect tumor cells that overexpress receptors specific for these peptides (see, e.g., Denzler and Reubi, Cancer 85(1):188-198, 1999). Moreover, somatostatin, bombesin and many other biologically active peptide agonist analogs are rapidly internalized after binding to their receptors (see, e.g., Lukinius et al, Acta Onc. 38:383-387, 1999). This internalization of the peptide analogs may result in translocation to the cell nucleus (Chen et al., Am. J. Physiol. Renal Physiol., 279: F440-F448, 2000).
Somatostatin analogs bind specific somatostatin receptor subtypes that are present on the surface of specific normal or diseased tissues. Somatostatin receptors are up-regulated in specific diseased tissues, including inflammatory bowel diseases, rheumatoid arthritis, and a variety of tumor types, as well as blood vessels supplying many tumors (Denzler and Reubi, Cancer, 85:4188-198, 1999). Similarly, receptors specific for another biologically active peptide, Substance P, can be up-regulated in various diseases.
At least five somatostatin receptors subtypes have been characterized, and tumors can express various receptor subtypes. (Shaer, et al., Int. 3. Cancer 70:530-537, 1997). Naturally occurring somatostatin and its analogs exhibit differential binding to these receptor subtypes, allowing precise targeting of a peptide analog to specific diseased tissues.
The physical and chemical properties of many cytotoxic agents make drug conjugation to biologically active peptides, such as somatostatin and bombesin, problematic. For example, the drug may reduce the specificity of binding or the biological activity of the peptide analog, limiting its effectiveness as a targeting agent. Additionally, therapeutic and cytotoxic agents may have chemical properties that promote accumulation of drug-peptide analogs in certain organs, increasing toxicity and reducing efficacy. Effective means to link cytotoxic agents to a targeting agent such as a biologically active peptide or an antibody, while retaining the activity of each component are needed to maximize therapeutic effects, while minimizing toxicity.